Diego Tonolla is a limnologist (freshwater researcher) at the aquatic research institute Eawag at the Swiss Federal Institute of Technology Zurich and a pioneer of this new method to help determine the ecological status of a watercourse. With his hydrophones (the technical term for the specialized microphones), he records sound patterns which, depending on the river in question, its currents and flow velocity, differ to such an extent that Tonolla can create an acoustic “fingerprint” for every body of water he examines. “Every river sounds different,” he explains, “yet we can identify typical sound patterns.” He is able to record them thanks to the special properties of the hydrophone, which converts underwater sounds into an electrical potential that corresponds to the sound pressure. This potential then creates sounds we perceive as droning or gushing. In the case of a straight riverbed and even flow velocity, we hear a muffled bass tone with low frequencies.

When the water shoots over a rise, medium frequencies increase. Waterfalls cause the tapestry of sound to become much more powerful, and when the stream or river returns to gliding leisurely along, slowed by curves and gravel banks, all that reaches our ears is a scarcely audible murmur. These sounds are converted into data consisting of volumes, frequencies and time units, which software then visualizes as colorful charts that provide insights into the ecological condition of the body of water. In this way, researchers can analyze and portray the condition of the entire course of a river. That is what Tonolla has done – aside from his status explorations of about 20 European (mostly Swiss) rivers – for example, with fellow scientists from the University of Montana. They drove a distance of 30 kilometers down the North Fork Flathead River in the Rocky Mountains and created an acoustic portrait of it.

Nathaniel Morse of the University of Vermont and other limnologists have become committed supporters of Diego Tonolla’s method. Tonolla advocates his approach mostly because it is “blindingly simple”. “Where there is more turbulence, i.e., more noise, more oxygen is absorbed.”

• Part 1: Acoustical monitoring of streams and rivers
• Part 2: More advantages of acoustical water-profiles